Dashboard creates the MQTT client and the HA device with the dummy
"Uptime" sensor (D41) in overlay and both desktop modes, connects in
started() and disconnects gracefully on aboutToQuit. The new platform
mode --desktop-novr behaves like --desktop for now (D22). The main tab
shows device, broker, connection state and last error (UI-1) through
the ConnectionStatus QML singleton.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
